The invention relates generally to the purification of dianhydrides and related bisimides. In one aspect the invention relates to the purification of dianhydrides and bisimides which are oxybisphthalic compounds. In a particular aspect, the invention relates to the purification of oxybisphthalic anhydrides.
Oxybisphthalic compounds, such as for example, oxybisphthalic anhydride (ODPA) are key raw materials used in preparation of polyetherimides. Bisimides related to ODPA, for example the N,N′-dimethylbisimide of ODPA, are useful intermediates in ODPA preparation. ODPA itself is a key monomer useful in the preparation of high temperature polyetherimides by condensation polymerization reaction with a diamine. In general, condensation polymerization processes require that the component monomers be of high purity in order to effectively build polymer molecular weight, obtain good reaction kinetics, and provide a thermally stable and processable polymer.
ODPA can be produced by the phase transfer catalyzed coupling of chlorophthalic anhydride in the presence of potassium carbonate and an organic phase transfer catalyst, such as hexaethylguanidinium chloride (HEGCl). Alternately, ODPA can be produced by hydrolysis of the related bisimide prepared in turn by phase transfer catalyzed coupling of an N-alkyl chlorophthalimide in the presence of potassium carbonate and an organic phase transfer catalyst, such as hexaethylguanidinium chloride (HEGCl). Although the coupling processes are efficient in the presence of the organic phase transfer catalyst, the product dianhydride or related bisimide may contain a significant fraction of the phase transfer catalyst employed, or its reaction and/or degradation products/adducts. Residual phase transfer catalyst has been shown to have an adverse effect on the thermal stability of polymers prepared using ODPA as a monomer or comonomer.
Therefore, there is a need for dianhydrides and related bisimides which are substantially free of residual phase transfer catalyst. Moreover, there is a need for more efficient methods for the separation of residual organic phase transfer catalysts from dianhydrides and related bisimides prepared using synthetic methods involving one or more reactions mediated by a phase transfer catalyst.